This invention relates to elastic substantially linear ethylene polymers having improved processability, e.g., low susceptibility to melt fracture, even under high shear stress conditions. Such substantially linear ethylene polymers have a critical shear rate at the onset of surface melt fracture substantially higher than, and a processing index substantially less than, that of a linear polyethylene at the same molecular weight distribution and melt index.
Molecular weight distribution (MWD), or polydispersity, is a well known variable in polymers. The molecular weight distribution, sometimes described as the ratio of weight average molecular weight (Mw) to number average molecular weight (Ma) (i.e. Mw/Mn) can be measured directly, e.g., by gel permeation chromatography techniques, or more routinely, by measuring I10/I2 ratio, as described in ASTM D-1238. For linear polyolefins, especially linear polyethylene, it is well known that as Mw/Mn increases, I10/I2 also increases.
John Dealy in xe2x80x9cMelt Rheology and Its Role in Plastics Processingxe2x80x9d (Van Nostrand Reinhold, 1990) page 597 discloses that ASTM D-1238 is employed with different loads in order to obtain an estimate of the shear rate dependence of melt viscosity, which is sensitive to weight average molecular weight (Mw) and number average molecular weight (Mn).
Bersted in Journal of Applied Polymer Science Vol. 19, page 2167-2177 (1975) theorized the relationship between molecular weight distribution and steady shear melt viscosity for linear polymer systems. He also showed that the broader MWD material exhibits a higher shear rate or shear stress dependency.
Ramamurthy in Journal of Rheology, 30(2),337-357 (1986), and Moynihan, Baird and Ramanathan in Journal of Non-Newtonian Fluid Mechanics, 36, 255-263 (1990), both disclose that the onset of sharkskin (i.e., surface melt fracture) for linear low density polyethylene (LLDPE) occurs at an apparent shear stress of 1-1.4xc3x97106 dyne/cm2, which was observed to be coincident with the change in slope of the flow curve. Ramamurthy also discloses that the onset of surface melt fracture or of gross melt fracture for high pressure low density polyethylene (HP-LDPE) occurs at an apparent shear stress of about 0.13 MPa (1.3xc3x97106 dyne/cm2). Ramamurthy also discloses that xe2x80x9cthe corresponding shear stresses (0.14 and 0.43 MPa) for linear polyethylenes are widely separated.xe2x80x9d However, these LLDPE resins are linear resins, and are believed to be those made by Union Carbide in their UNIPOL process (which uses conventional Ziegler-Natta catalysis which results in a heterogeneous comonomer distribution). The LLDPE is reported in Tables I and II to have a broad Mw/Mn of 3.9. The melt fracture tests conducted by Ramamurthy were in the temperature range of 190 to 220 C Furthermore, Ramamurthy reports that the onset of both surface and gross melt fracture (for LLDPE resins) are xe2x80x9c . . . essentially independent of MI (or molecular weight), melt temperature, die diameter (0.5-2.5 mm), die length/diameter ratio (2-20), and the die entry angle (included angle: 60-180 degrees).xe2x80x9d
Kalika and Denn in Journal of Rheology, 31, 815-834 (1987) confined the surface defects or sharkskin phenomena for LLDPE, but the results of their work determined a critical shear stress at onset of surface melt fracture of 0.26 MPa, significantly higher than that found by Ramamurthy and Moynihan et al. Kalika and Denn also report that the onset of gross melt fracture occurs at 0.43 MPa which is consistent with that reported by Ramamurthy. The LLDPE resin tested by Kalika and Denn was an antioxidant-modified (of unknown type) UNIPOL LLDPE having a broad Mw/Mn of 3.9. Kalika and Denn performed their melt fracture tests at 215 C. However, Kalika and Denn seemingly differ with Ramamurthy in the effects of their L/D of the rheometer capillary. Kalika and Denn tested their LLDPE at L/D""s of 33.2, 66.2, 100.1, and 133.1 (see Table 1 and FIGS. 5 and 6).
International Patent Application (Publication No. WO 90/03414) published Apr. 5, 1990 to Exxon Chemical Company, discloses linear ethylene interpolymer blends with narrow molecular weight distribution and narrow short chainbranching distributions (SCBDs). The melt processibility of the interpolymer blends is controlled by blending different molecular weight interpolymers having different narrow molecular weight distributions and different SCBDs.
Exxon Chemical Company, in the Preprints of Polyolefins VII International Conference, page 45-66, Feb. 24-27 1991, disclose that the narrow molecular weight distribution (NMWD) resins produced by their EXXPOL(trademark) technology have higher melt viscosity and lower melt strength than conventional Ziegler resins at the same melt index. In a recent publication, Exxon Chemical Company has also taught that NMWD polymers made using a single site catalyst create the potential for melt fracture (xe2x80x9cNew Specialty Linear Polymers (SLP) For Power Cables,xe2x80x9d by Monica Hendewerk and Lawrence SpenadeL presented at IEEE meeting in Dallas, Tex. September, 1991). In a similar vein, in xe2x80x9cA New Family of Linear Ethylene Polymers Provides Enhanced Sealing Performancexe2x80x9d by Dirk G. F. Van der Sanden and Richard W. Halle, (February 1992 Tappi Journal), Exxon Chemical Company has also taught that the molecular weight distribution of a polymer is described by the polymers melt index ratio (i.e., I10/I2 and that their new narrow molecular weight distribution polymers made using a single site catalyst are xe2x80x9clinear backbone resins containing no functional or long chain branches.xe2x80x9d
U.S. Pat. No. 5,218,071 (Canadian patent application 2,008,315-A) to Mitsui Petrochemical Industries, Ltd., teaches ethylene copolymers composed of structural units (a) derived from ethylene and structural units (b) derived from alpha-olefins of 3-20 carbons atoms, said ethylene copolymers having [A] a density of 0.85 -0.92 g/cm3, [B] an intrinsic viscosity as measured in decalin at 135 C of 0.1-10 dl/g, [C] a ratio (Mw/Mn) of a weight average molecular weight (Mw) to a number average molecular weight (Mw) as measured by GPC of 1.2-4, and [D] a ratio (MFR810/MFR2) of MFR under a load of 10 kg to MRF2 under a load of 2.16 kg at 190 C of 8-50, and being narrow in molecular weight distribution and excellent in flowability. However, the ethylene copolymers of U.S. Pat. No. ""071 are made with a catalysis system composed of methylaluminoxane and ethylenebis(indenyl)hafnium dichloride (derived from HfCl4, containing 0.78% by weight of zirconium atoms as contaminates). It is well known that mixed metal atom catalyst species (such as hafnium and zirconium in U.S. Pat. No. ""071) polymerizes copolymer blends, which are evidence by multiple melting peaks. Such copolymer blends therefore are not homogeneous in terms of their branching distribution.
WO 85/04664 to BP Chemicals Ltd. teaches a process for the thermo-mechanical treatment of copolymers of ethylene and higher alpha-olefins of the linear low density polyethylene type with at least one or more organic peroxides to produce copolymers that are particularly well suited for extrusion or blow-molding into hollow bodies, sheathing, and the like. These treated copolymers show an increased flow parameter (I21/I2) without significantly increawsing the Mw/Mn. However, the novel polymers of the present invention have long chained branching and obtained this desirable result without the need of a peroxide treatment U.S. Pat. No. 5,096,867 discloses various ethylene polymers made using a single site catalyst in combinations with methyl aluminoxane. These polymers, in particular Example 47, have extremely high levels of aluminum resulting from catalyst residue. When these aluminum residues are removed from the polymer, the polymer exhibits gross melt fracture at a critical shear stress of less than 4xc3x97106 dyne/cm2.
All of the foregoing patents, applications, and articles are herein incorporated by reference.
Previously known narrow molecular weight distribution linear polymers disadvantageously possessed low shear sensitivity or low I10/I2 value, which limits the extrudabdity of such polymers. Additionally, such polymers possessed low melt elasticity, causing problems in melt fabrication such as film forming processes or blow molding processes (e.g., sustaining a bubble in the blown film process, or sag in the blow molding process etc.). Finally, such resins also experienced melt fracture surface properties at relatively low extrusion rates thereby processing unacceptably.
A new class of homogeneous ethylene polymers have now been discovered which have long chain branching and unusual but desirable bulk properties. These new polymers include both homopolymers of ethylene and interpolymers of ethylene and at least one alpha-olefin. Both the homo- and interpolymers have long chain branching, but the interpolymers have short chain branching in addition to the long chain branching. The short chain branches are the residue of the alpha-olefins that are incorporated into the polymer backbone or in other words, the short chain branches are that part of the alpha-olefin not incorporated into the polymer backbone. The length of the short chain branches is two carbon atoms less than the length of the alpha-olefin comonomer. The short chain branches are randomly, i.e. uniformity, distributed throughout the polymer as opposed to heterogeneously branched ethylenelalpha-olefin interpolymers such as conventional Zeigler LLDPE.
These novel ethylene polymers have a shear thinning and ease of processability similar to highly branched low density polyethylene (LDPE), but with the strength and toughness of linear low density polyethylene (LLDPE). These novel ethylene polymers can also be characterized as xe2x80x9csubstantially linearxe2x80x9d polymers, whereby the bulk polymer has an average of up to about 3 long chain branches/1000 total carbons or in other words, at least some of the polymer chains have long chain branching. The novel substantially linear ethylene polymers are distinctly different from traditional Ziegler polymerized heterogeneous polymers (e.g., LLDPE) and are also different from traditional free radical/high pressure polymerized LDPE. Surprisingly, the novel substantially linear ethylene polymers are also different from linear homogeneous ethylene polymers having a uniform comonomer distribution, especially with regard to processability.
These novel ethylene polymers, especially those with a density greater than or equal to about 0.9 g/cm3 are characterized as having:
a) a melt flow ratio, I10/I2, xe2x89xa75.63,
b) a molecular weight distribution , defined by the equation:
Mw/Mnxe2x89xa6(I10/I2)xe2x88x924.63,
c) a critical shear stress at onset of gross melt fracture greater than about 4xc3x97106 dyne/cm2, and
d) a single melt peak as determined by differential scanning calorimetry (DSC) between xe2x88x9230 and 150 C.
The novel ethylene polymers can also be characterized as having:
a) a melt flow ratio, I10/I2, xe2x89xa75.63,
b) a molecular weight distribution, Mw/Mn, defined by the equation:
MIw/Mnxe2x89xa6(I10/I2)xe2x88x924.63,
c) a critical shear rate at onset of surface melt fracture at least 50 percent greater than the critical sear rate at the onset of surface melt fracture of a linear ethylene polymer with an I2, Mw/Mn, and density each within ten percent of the novel ethylene polymer, and
d) a single melt peak as determined by differential scanning calorimetry (DSC) between xe2x88x9230 and 150 C.
In another aspect, the novel ethylene polymers, especially those having a density greater than or equal to about 0.9 g/cm3, are characterized as having:
a) a melt flow ratio, I10/I2, xe2x89xa75.63, and
b) a molecular weight distribution, Mw/Mn of from about 1.5 to about 2.5,
c) a critical shear stress at onset of gross melt fracture greater than about 4xc3x97106 dyne/cm2, and
d) a single melt peak as determined by differential scanning calorimetry (DSC) between xe2x88x9230 and 150 C.
In still another aspect, the novel ethylene polymers are characterized as having:
a) a melt flow ratio, I10/I2, xe2x89xa75.63,
b) a molecular weight distribution, Mw/Mm of from about 1.5 to about 2.5,
c) a critical shear rate at onset of surface melt fracture of at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear ethylene polymer with an I2, Mw/Mn, and density each within ten percent of the novel ethylene polymer, and
d) a single melt peak as determined by differential scanning calorimetry (DSC) between xe2x88x9230 and 150 C.
The substantially linear ethylene polymers can also be characterized as having a critical shear rate at onset of surface melt fracture of at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear ethylene polymer having an 12, Mw/Mn and density each within ten percent of the substantially linear ethylene polymer.
In still another aspect the novel polymer can be characterized as a substantially linear ethylene bulk polymer having:
(a) and average of about 0.01 to about 3 long chain branches/1000 total carbons,
(b) a critical shear stress at onset of gross melt fracture of greater than about 4xc3x97106 dyne/cm2, and
(c) a single DSC melt peak between xe2x88x9230 and 150 C.
The substantially linear ethylene bulk polymer can also be characterized as having:
(a) an average of about 0.01 to about 3 long chain branches/1000 total carbons,
a critical shear rate at onset of surface melt fracture of at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear ethylene polymer having an I2, Mw/Mn and density each within ten percent of the substantially linear ethylene bulk polymer, and
(c) a single DSC melt peak between xe2x88x9230 and 150 C.
In still another aspect, the ethylene polymer can be characterized as a substantially linear ethylene bulk polymer having:
(a) an average of about 0.01 to about 3 long chain branches/1000 total carbons,
(b) a melt flow ratio, I10/I2,  greater than 5.63,
(c) a molecular weight distribution, Mw/Mn, from about 1.5 to about 2.5, and
(d) a single DSC melt peak between xe2x88x9230 and 150 C.
The novel ethylene polymers, especially the substantially linear ethylene polymers, also have a processing index (PI) less than or equal to about 70 percent of the PI of a linear ethylene polymer at about the same I2, Mw/Mn, and density each within ten percent of the novel ethylene polymer.
Compositions comprising the novel ethylene polymer and at least one other natural or synthetic polymer are also within the scope of the invention.
Elastic substantially linear ethylene polymers comprising ethylene homopolymers or an interpolymer of ethylene with at least one C3-C20 alpha-olefin copolymers are especially preferred.